The present invention relates to polymer strap and methods of producing the same.
Steel and plastic strapping are used for a wide variety of applications, often to secure very large coils of steel, synthetic fiber bales and heavy palletized boxes. Steel strapping has advantages in that it has high strength and temperature resistance and outstanding creep resistance. Steel strapping is typically used on heavy loads where high strap strengths and low creep properties are required. However, steel strapping, although very useful in maintaining the quality of the packaging, may be more difficult to dispose of and the strap can have sharp edges.
Plastic strap has found particular application to lower strength packaging requirements and represents a less expensive alternative to steel strap. Plastic strap typically has an elastic behavior within limits which allows the strap to remain tight on a package even if the package collapses or somewhat consolidates. The plastic strap is easily disposed of and is safer to use than steel strap because it does not have the dangerous sharp edges of steel strap. Plastic strap is made initially from an extruded strand or sheet, and the polymeric material comprising the extrusion is then oriented, a process which increases the strength of the plastic material to approximately 10 times the unorientated strength of the material.
Plastic strap in the market place has generally been limited to applications where a tensile strength of no greater than 65,000 psi is required. Some strap manufacturers have claimed to achieve plastic strap strengths in excess of 75,000 psi using PET after a two-stage stretching technique. See U.S. Pat. No. 4,022,863 to Karass et al. However, the workability and operability of the Karass process is questionable, particularly at the moisture levels discussed in the Karass patent.
The creep properties of plastic strapping and the difficulty of securing plastic strap have limited the applications of plastic strapping. The ability to provide a weld with plastic strap is also a limitation which prevents plastic strapping from being used in higher break strength applications. It should be noted that high strength plastic fibers are known, but strapping which combines these fibers with an appropriate support substrate has limited applications.
One method of manufacturing plastic strap according to the present invention includes initially forming a molecularly oriented crystalline polyester or polyester copolymer of uniform cross-section which is many times wider than the thickness thereof. The material is oriented by longitudinal stretching at a ratio of at least 5.0 to 1.0. The longitudinal stretching may be carried out at raised temperatures with the material being maintained at this stressed condition until sufficient cooling has taken place. However, for strap with a combination of high strength and low brittleness, the third stretching stage should be conducted at temperatures at or below about 200.degree. F. It has been found that the tensile strength of the strap can be significantly increased to at least 70,000 psi.
In one embodiment of the invention, an outer layer of amorphous material is formed on at least one and preferably both sides of the material. The outer layer may be imparted after the material has been extruded and oriented by stretching. This is done by heating the outside surface of the material by, for example, exposing the material to heated rolls. Heating the oriented material causes the molecular structure to lose its orientation, at least in part, to a depth which depends upon the extent to which the temperature of the material is raised. To control brittleness of the core, the material may be chilled just prior to the surface treatment, and may be chilled again just after the surface treatment. The outer layer is preferably unitary and integral with the underlying high strength layer, and preferably has a thickness of at least about 0.0005 inches. Strap made from material formed by the process of the present invention includes a high strength underlying layer which is highly oriented. The strap, when fused to itself via the outer layers, has a joint whose break strength is a function of the break strength of the high strength layer. The outer layer has been found to provide the strap with substantially improved corner strength. The outer layer has been found to resist crack initiation and propagation in the welded strap sections. It has been found with the strap of the present invention that the conventional hot knife welding or fusing techniques can be used to provide a weld between strap sections resulting in a connection having an overall break strength approaching the break strength of at least about 80% of the underlying high strength layer.
The formation of an outer amorphous surface by melting the extruded material may have substantial benefit even on strap of moderate strength. The melting of the outer surface improves the weldability of the strap to such an extent that it may be economically feasible to use the surface treatment aspects of the present invention without taking particular advantage of any potentially high strength of the core material.